Investigation of ancient mass movements by seismic noise analysis: application to the Romanian Carpathian Mountains

Assessing the geometry and volume of mass movements is essential for the appraisal of slope stability and for the understanding of slope failure trigger mechanisms. For the latter, we developed seismic ambient noise measurement techniques in order to better characterize the sub-surface of ancient deep-seated landslides in seismic regions as the Carpathian Mountains in Romania.
In particular, we conducted thorough seismological and geophysical campaigns on the landslides of Eagle’s Lake, Paltineni, and Varlaam, in the Buzau-Vrancea region, Romania. This region, marked by a high seismicity with intermediate-depth earthquakes, hosts very large and generally old (i.e. >1000 years) mass movements with morphologies which might be due to seismically induced failure.

On the three study sites, we performed abundant horizontal-to-vertical noise spectral ratio (HVSR) measurements and installed several seismic arrays. The HVSR technique, based on the analysis of three component seismic signals, is commonly used to identify the resonance frequency of a given site. Polarization of the seismic wavefield is also investigated over the landslides. Through the installation of seismic arrays, we analyse the dispersive properties of the surface waves. By jointly inverting the information through a non-linear approach, we retrieve the shear-wave velocity profiles beneath the arrays and identify velocity contrasts with depth.
On Eagle’s Lake and Paltineni rockslides, the results have also been integrated with seismic refraction tomography profiles, evidencing lateral contrasts in soil properties, and multichannel analysis of surface waves providing the subsurface shear-wave velocities. At Varlaam, the extensive measurements performed over the landslide allowed us to identify a major impedance contrast at depth highlighting the base of the failed body. We also performed UAV flights to establish a 3D model of the investigated sites. All these investigations contributed to assess the landslide geometries and estimate the volumes of the failed bodies.

This work aims, in prospect, at reconstructing the conditions and the energy needed for triggering these landslides in order to understand if a seismic component is applicable in the failure process.